1. Field of the Invention
The present invention is directed to inkjet printers and, more particularly, to a method for controlling an inkjet printing device operating in accordance with the continuous inkjet principle.
2. Description of the Related Art
Inkjet printing devices which do not use printing plates are increasingly used in plate-based printing presses, mainly printing presses working on the principle of offset printing, e.g., in web-fed rotary offset printing presses and sheet-fed printing presses. These plateless inkjet printing devices are used in-line with offset printing particularly for individualizing printed products produced by offset printing, such as with barcodes, numbering or other markings. Inkjet printing devices of this kind have at least one inkjet printing head.
Inkjet printing devices working according to the continuous inkjet principle supply ink droplets at a defined, constant droplet-generating frequency usually on the order of magnitude of between 10 Hz and several hundred kHz. In order to control an inkjet printing device working in accordance with the continuous inkjet principle such that a print image to be printed by the inkjet printing device is exactly positioned on a substrate to be imprinted, the movement of the substrate is monitored by a sensor. The monitoring is typically performed by a displacement transducer designed as an encoder, and the signal supplied by the sensor is used, according to the prior art, to control the inkjet printing device. The frequency of the signal supplied by the sensor, particularly by the encoder, depends on the transporting speed of the substrate to be imprinted.
Accordingly, in practice, there are two frequencies which are independent from one another, i.e., on the one hand, the droplet generating frequency of the inkjet printing device and, on the other hand, the frequency of the signal supplied by the sensor which is obtained from monitoring the movement of the substrate to be imprinted. A good printing quality can be provided at a low transporting speed of the substrate, i.e., when the droplet generating frequency is greater than the frequency of the signal supplied by the sensor. On the other hand, when the transporting speed of the substrate to be imprinted is high, i.e., when the droplet generating frequency is approximately on the order of the frequency of the signal supplied by the sensor, interference effects or beat effects can occur in the print image which are visible as light or dark fringes or stripes in the print image to be printed by the inkjet printing device and which impair the printing quality. Up to the present time, there has been no known method for controlling an inkjet printing device operating according to the continuous inkjet principle which remedies this problem.